Is it possible to make is so that 1 culture uses random faces, i.e. Gothamite can come in arab, white, black,etc.
'Fraid not. A culture is associated with a single graphical_culture, which will have a single skin color. That means randomly generated characters of that culture will all use the same faces (altered by DNA string, which will not affect skin color, just stuff like hair, beard, cheeks, mouth, etc.)
The only exception to that would be event generated characters, where you can apply the new set_graphical_culture command to give a different graphical culture to the created character.